1. Field of the Invention
This invention relates to improvements in an ultrasonic measurement method and apparatus for subjecting an object to an ultrasonic transmission, and receiving reflected ultrasonic waves from the interior of the object to measure the acoustic characteristics of the interior. More particularly, the invention relates to an ultrasonic measurement method and apparatus for providing information relating to the frequency dependence of ultrasonic attenuation within the object.
2. Description of the Prior Art
Ultrasonic measurement techniques find application widely in such fields as material testing, sonar and medical diagnosis. In particular, ultrasound scanner systems for medical purposes have recently been developed.
The principle of operation of an ultrasound scanner system resides in use of a pulse-echo method and utilizes a phenomenon wherein an ultrasonic pulse transmitted into a living body, which is the object undergoing measurement, is reflected at a boundary where there is a difference in acoustic impedence in living tissue. The reflected wave (echo) is received and processed to display a tomograph of the living body by a so-called B-mode method. The echo therefore contains information relating to the scattering and reflection characteristic of the region where the echo is produced and the round-trip attenuation characteristic along the propagation path, as well as such information as the degree of ultrasonic attenuation, acoustic impedance and propagation velocity of sound, all in living tissue. Despite such a variety of effective information contained in the echo, however, the information being utilized at the present time is solely the amplitude of the echo.
More specifically, in the present state of the art, the propagation velocity of sound in living tissue is assumed to be constant and, with regard to attenuation ascribable to ultrasonic propagation, the value of the echo amplitude obtained by an arbitrary correction performed by a so-called STC (sensitivity time control) circuit is displayed as a mere tomograph on a cathode-ray tube following brightness modulation. This is referred to as a "B-mode display". Accordingly, the tomograph obtained is nothing more than a qualitatively imaged two-dimensional distribution at acoustic impedence boundaries in living tissue, so that morphological information relating to the position and shape of the biological tissue inevitably forms the core of the information utilized. However, the state of the art is such that biological tissue characteristics such as degree of attenuation and propagation velocity of sound are not measured.
Several attempts at attaining attenuation information relating to biological tissue have been reported. However, as will be described below in further detail, an echo signal contains two types of information, namely attenuation due to propagation through biological tissue, and intensity of reflection at an interface or boundary where there is a difference in acoustic impedence. Both of these quantities are unknown. Therefore, isolating the effects of these two quantities is extremely difficult at the present time.
If the reflected intensity is assumed to be independent of the frequency of the ultrasonic waves and ultrasonic waves having two or more frequencies are transmitted and the ultrasonic echo is received with regard to the same portion of the object under measurement followed by measuring the sound pressure ratio of each frequency component of the echo, then it will be possible to eliminate the influence of the reflected intensity and derive an attenuation coefficient. See the specification of Japanese Patent Application Laid-Open No. 49-38490 in this connection. The foregoing assumption holds in the case of an acoustic interface having a sufficiently wide spread in comparison with the wavelength of the ultrasonic waves, e.g. in the case of a planar reflector. With actual biological tissue, however, scatterers of a size approximately equivalent to or smaller than the wavelengths used are also considered to be present. Therefore, the foregoing assumption will not necessarily hold true for the entirety of a biological tissue.
If it is assumed that the reflected intensity is approximately constant at a certain portion of a biological tissue, then one may consider that the echo sound pressure ratio across the front and back of this portion of the tissue is proportional to the attenuation coefficient. Further, several theories have been brought forward wherein an attenuation coefficient is obtained by presupposing a function for the frequency dependence of the reflected intensity, transmitting ultrasonic waves having three or more frequencies, receiving the ultrasonic echos from the same portion of the object under measurement, and measuring the sound pressure of each frequency component of the echos, with the attenuation coefficient being obtained from the sound pressure. See the specification of Japanese Patent Application Laid-Open No. 56-147082.
Thus, the method employed to isolate and measure an attenuation coefficient in all of the foregoing cases involves making an assumption with regard to the reflected intensity, as well as transmitting and receiving ultrasonic waves having a single frequency component or a plurality of frequency components.
Several attempts to measure degree of attenuation combining attentuation and reflection (scattering information have been reported (Japanese Patent Application Laid-Open Nos. 58-24824 and 57-179745), and there have been reports of attempts to measure the frequency dependence of the degree of attenuation (Japanese Patent Application Laid-Open Nos. 57-89858, 57-139326, 57-209040). Basically, these reports disclose performing frequency analysis by well-known Fourier analysis or the like with regard to an echo waveform (signal) at a region of interest in biological tissue, and displaying the obtained frequency spectrum together with the conventional tomograph. These approaches display a difference between spectra at two points in the region of interest to obtain information relating to the frequency dependence of the acoustic characteristic (attenuation) in the region of interest. In any case, the items of information obtained are spectra and the diagnosis of tissue properties is performed by ascertaining any change in the shape of the spectra themselves in comparison with image information, namely the conventional tomograph. Performing diagnosis in such fashion involves numerous problems in actual practice. Moreover, one can conjecture that problems will also be encountered in ascertaining in real-time the distribution of a spectrum with respect to the spread of the tissue.
A well-known method of displaying tomographs of different frequencies obtained by multifrequency tomography is referred to as the "Spectra Color" method. The details of this method are reported in "Imaging of Invisible Information" compiled by the Institute of Television Engineers of Japan, pp. 195-196, published by Shokodo K.K., in Lecture Discourses 28-27 (pp. 47-48) of the 28th meeting (November, 1975) of the Japan Society of Ultrasonics in Medicine, and in the specification of U.S. Pat. No. 4,228,804. Though these methods attempt to ascertain the frequency dependence of biological tissue attenuation information by using different frequencies, they do nothing more than provide a superimposed display of the tomographs for the respective frequencies used. Accordingly, a fundamental problem encountered with these methods is the fact that a tissue which by nature has a certain attenuation with respect to ultrasonic waves exhibits a measured attenuation value which is different owing to the thickness of the attenuating medium (tissue), the distance from the ultrasonic transducer to the medium (tissue) to be examined, and the influence of other tissues lying between the transducer and the tissue to be examined.